The assembly of immunoglobulin-like modules in titin: Implications for muscle elasticity

Titin, a giant muscle protein, forms filaments that span half of the sarcom ere and cover, along their length, quite diversified functions. The region of titin located in the sarcomere I-band is believed to play a major role i n extensibility and passive elasticity of muscle. In the I-band, the titin sequence contains tandem immunoglobulin-like (Ig) modules intercalated by a potentially non-globular region. By a combined approach making use of smal l angle X-ray scattering and nuclear magnetic resonance techniques, we have addressed the questions of what are the average mutual orientation of poly -Igs and the degree of flexibility around the domain interfaces. Various re combinant fragments containing one, two and four titin I-band tandem domain s were analysed. The small-angle scattering data provide a picture of the d omains in a mostly extended configuration with their long axes aligned head -to-tail. There is a small degree of bending and twisting of the modules wi th respect to each other that results in an overall shortening in their max imum linear dimension compared with that expected for the fully extended, l inear configurations. This shortening is greatest fur the four module const ruct (approximate to 15%). N-15 NMR relaxation studies of one and two-domai n constructs show that the motions around the interdomain connecting region s are restricted, suggesting that titin behaves as a row of beads connected by rigid hinges. The length of the residues in the interface seems to be t he major determinant of the degree of flexibility. Possible implications of our results for the structure and function of titin in muscles are discuss ed. (C) 1998 Academic Press.